Highlights

Potential for structural and stratigraphic Triassic–Jurassic, Cretaceous and Paleogene plays

Special Notices apply, refer to Guidance Notes

Release Areas AC16-1 and AC16-2 are located in the Vulcan Sub-basin, western Bonaparte Basin in the Timor Sea (Figure 1), approximately 255 km offshore and 600 km from Darwin. Both Release Areas cover two graticular blocks (170 km2 each) in water depths up to 210 m. The Release Areas are located on the flank of the Paqualin Graben extending onto the Jabiru Terrace, and are adjacent to a number of discoveries including the Oliver oil and gas accumulation and the Pengana gas accumulation.

The Vulcan Sub-basin is a northeast-oriented Mesozoic extensional depocentre that contains Permian–Holocene sediments (Figure 2). The sub-basin comprises a complex series of horsts, graben and marginal terraces. It is a proven hydrocarbon province which includes several producing oil fields of the Montara project, as well as several other oil fields (Jabiru, Cassini and Challis) and sub-economic accumulations. Development options for the Cash/Maple gas and oil accumulation and further production of the Puffin oil accumulation are currently being considered. The Release Areas are located on the flank of the Paqualin Graben extending onto the Jabiru Terrace, and are adjacent to a number of discoveries including the Oliver oil and gas accumulation and the Pengana gas accumulation.

The Swan and Paqualin graben form the major source kitchens in the Vulcan Sub-basin, containing mature oil- and gas-prone source rocks in the lower Vulcan Formation, Montara Formation and underlying Plover Formation (Figure 2).

Release Areas AC16-1 and AC16-2 are proximal to the Cash/Maple gas accumulation currently being planned for development by PTTEP. The nearest fixed infrastructure is the Montara platform, 120 km to the southwest, and the Ichthys pipeline, under construction 170 km to the southeast. In the past, production from oil fields in the Vulcan Sub-basin has been through the use of floating, production, storage and offtake (FPSO) vessels. The port of Darwin is 630 km from the release areas.

Critical risks

Structural complexity in the Vulcan Sub-basin has meant adequately imaging reservoir, trap and seals has been challenging. Repeated episodes of fault reactivation (particularly during the Neogene) and associated remigration, water washing and gas flushing have locally degraded oil accumulations in ‘leaky traps’ such as Jabiru. The risk of water washing degrading any oil accumulations in the Release Areas is difficult to predict. For example, the Jabiru and Tenacious oils show moderate water washing, Audacious 1 oil shows significant water washing, and the Oliver 1 oil shows no evidence of water washing (Edwards et al, 2004). The Delamere 1 gas accumulation near release area AC16-2 is biodegraded. While remobilised evaporites, as seen at Paqualin 1, are less extensive in the Vulcan Sub-basin than in other parts of the Bonaparte Basin, there is a risk that localised salt intrusions could impact on seal integrity. Elevated geothermal gradients during the Middle Jurassic, as modelled in Jabiru 1, may affect the timing of hydrocarbon generation and expulsion in the Vulcan Sub-basin and adversely affect reservoir quality (Colwell and Kennard, 1996).

Marine and environmental information

Release area AC16-1

Located in the Timor Sea, 250 km from the Western Australia coastline in water depths of 110230 m.

Sandy sediments at the seabed with no terrigenous sediment input.

Overlies the Vulcan Sub-basin at the continental shelf break.

Not located within 50 km of any Commonwealth Marine Reserves.

Potential hazards: none identified.

Release area AC16-2

Located in the Timor Sea, 235 km from the Western Australia coastline in water depths of 100120 m.

Sandy sediments at the seabed with no terrigenous sediment input.

Overlies the Vulcan Sub-basin over generally flat continental shelf.

Not located within 50 km of any Commonwealth Marine Reserves.

Potential hazards: none identified.

Characteristics of the areas

Release Area AC16-1 is located in water depths of ~110–230 m, while Release Area AC16-2 is located in water depths of ~100–120 m, both overlying the Vulcan Sub-basin in the inboard section of either the Big Bank or Sahul Shoals region. Closest to these areas are the Halimeda and coral-rich carbonate banks of Jabiru Shoals, located approximately 5 km directly west of Release Area AC16-1. In this region of the Sahul Shoals, away from the carbonate-dominated banks and shoals, terrigenous sediments are absent, the seabed is generally flat and tidally influenced (Wienberg et al, 2010; Porter-Smith et al, 2004).

BEARDSMORE, G.R. AND O’SULLIVAN, P.B., 1995—Uplift and erosion on the Ashmore Platform, North West Shelf; conflicting evidence from maturation indicators. The APEA Journal, 35(1), 333–343.

BOTTEN, P.R., ROSSER, J. AND GORTER, J., 1992—Comparisons in the structural and stratigraphic evolution of the Vulcan Graben and basins in the eastern Timor Sea and ramifications for hydrocarbon exploration. Annual Meeting Abstracts - American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists 1992, 11–12.

HILLIS, R.R., MILDREN, S.D., PIGRAM, C.J. AND WILLOUGHBY, D.R., 1997—Rotation of horizontal stresses in the Australian North West continental shelf due to the collision of the Indo-Australian and Eurasian plates. Tectonics 16, 323–335.

SAQAB, M.M. AND BOURGET, J. 2015—Controls on the distribution and growth of isolated carbonate build-ups in the Timor Sea (NW Australia) during the Quaternary. Marine and Petroleum Geology, 62, 123–143.

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